The bid expiry feature allows you to select a time that your bid will expire, preventing the business from accepting your bid. This feature was released to offer Operators the choice of having their bid expire in the event that they need a certain amount of notice prior to bid acceptance. Here's an example of what this looks like:
You have the option of setting your expiration time as early as 24 hours prior to the Op start time and as late as 1 hour before the start time. If you are bidding on an Op and the start time is less than an hour away, you will not be able to set a bid expiry time.